Gypsum-plaster and filling compositions are generally prepared at the point of use by combining gypsum mortars, i.e. dry mixtures comprising not less than 10 weight percent gypsum, based on the total dry weight of the mortar, with an amount of water sufficient to enable the application of the resultant plaster or filling composition to a surface prior to it setting and hardening upon drying.
Conventional plaster and filling compositions often comprise a combination of gypsum and one or more additives such as, for example, thickeners, retarding agents, accelerating agents, dispersing agents, plasticizers and stabilizers, thereby providing a modified gypsum plaster or filling composition with improved physical characteristics. In particular, wetting agents or water retention additives may be utilized to impart a beneficial effect upon the physical characteristics of the plaster or filling compositions and/or the resultant hardened product, which are highly dependent upon both the initial hydration process and the subsequent water removal as the composition sets and hardens upon drying. For example, the incorporation of wetting agents or water retention additives may lead to a beneficial effect upon processing properties such as viscosity, open time, setting rate and drying time.
The use of polysaccharide derivatives, in particular water soluble cellulose ether compounds, as water retention additives in gypsum mortars is well known. For example, US-A-2004/0258901 discloses a gypsum plaster comprising a water soluble cellulose ether binder that has a preferred molecular weight between 12,000 and 30,000. In addition, US-A-2003/0005861 discloses a dry gypsum based mortar formulation modified with water-redispersible polymer powders for use in the construction industry. The thickeners suitable for use in this formulation include polysaccharides such as cellulose ethers. Further, EP-A-0774445 discloses a lime-containing gypsum plaster composition that uses a combination of a non-ionic cellulose ether and carboxymethylcellulose as the water retaining agent and thickener. The combination of these additives results in a prolonged open time.
In addition to cellulose ether, gypsum plaster compositions are known to contain other additives. For example, US-A-2005/0241541 discloses a mixture composition for use in preparing modified gypsum plasters, wherein the resultant plaster comprises 0.01 to 1.0 weight percent alkylhydroxyalkyl cellulose and/or hydroxyalkyl cellulose prepared from raw cotton linters and 0.0001 to 10 weight percent at least one additive. The list of specific additives generically disclosed includes, inter alia, gelatin.
When cellulose ether particles included in such gypsum mortars become solublized upon combination with water, the composition is thickened. As a consequence of such thickening, the aqueous phase tends not to wet the gypsum particles uniformly. Such non-uniform wetting of the gypsum particles can cause agglomeration of the components in the composition, resulting in the formation of visual defects such as clusters, lumps or nodules, collectively known as agglomerate structures, which become particularly apparent after the application of such compositions to a substrate, which may be either by spraying in the case of machine application or by hand. Such agglomeration can provide an unsatisfactory finish to the hardened composition, and consequently, once identified, can necessitate removal and intensive reworking of the wet gypsum composition to attempt to remove or reduce such agglomerate structures in the composition or smoothing the dry hardened surface by, e.g. sanding.
The problem of agglomeration in gypsum compositions comprising cellulose ethers is discussed in WO 99/64368, wherein an additive is used consisting mainly of cellulose ether and small amounts of polymerized carboxylic acid and a methacrylate or acrylate homo- or interpolymer. However, the use of aqueous carboxylic acid solutions can lead to a pH induced chain degradation of the cellulose ether. The problem of agglomeration is also disclosed in WO2009/018876, wherein an alkyl hydroxyalkycellulose is treated with a crosslinking agent, such as monoaldehydes, dialdehydes, e.g. glyoxal, and silicon compounds. However, the crosslinking reaction is dependent upon the pH of the resultant composition, with a high pH inducing rapid cleavage of the cross-linking. Accordingly, the pH of the composition must be carefully controlled in order that the crosslinking reaction occurs on a timeframe suitable for reducing agglomeration.
Whilst the problem of agglomeration in such modified gypsum plaster and/or filling compositions is addressed to some extent in the above prior art references, the solutions presented by the prior art rely upon the compositions having a high or low pH, which may be detrimental to the composition or suitability for application. Accordingly, there remains a desire to obtain a reduction in agglomeration within cellulose ether containing plaster or filler compositions without causing any significant detrimental effects on the performance or properties of the cellulose ether-containing gypsum compositions.